Extruded synthetic resinous foams are useful materials for many applications including thermal insulation, decorative purposes, packaging and the like. Extruded foams are generally made by melting a polymer with any other additives to create a polymer melt, mixing a blowing agent with the polymer melt at an appropriate temperature and pressure to produce a mixture whereby the blowing agent becomes soluble, i.e. dissolves, in the polymer melt. This mixture may then be extruded into a zone of reduced pressure so that the blowing agent becomes insoluble in the polymer melt and converts into a gas. As the blowing agent converts to a gas, bubbles are produced within the polymer melt. At this point, the polymer melt is cooled thereby producing a foam structure comprising closed cells resulting from the expansion of the blowing agent.
Traditional blowing agents used for extruded foam manufacture include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). One of the advantages of CFC and HCFC blowing agents is their high solubility in a polymer melt. Higher blowing agent solubility promotes viscosity reduction when mixed with the polymer melt. In turn, lower viscosity leads to lower energy requirements for mixing. A major disadvantage to traditional blowing agents is that an increasing number of governments worldwide have mandated the elimination of CFCs and HCFCs blowing agents due to growing environmental concerns.
Accordingly, there has been a movement to replace traditional blowing agents in favor of more environmentally friendly blowing agents such as hydrofluorocarbons. Unfortunately, hydrofluorocarbons (HFCs), and in particular tetrafluoroethane (134a), have a lower solubility in polymer melts compared to traditional blowing agents. The present invention is directed, in part, to increasing the solubility of 134a in a polymer melt by addition of an alcohol.
U.S. Pat. No. 5,182,308 issued to Volker et al. ("Volker") discloses a laundry list of blowing agent compositions some of which include HFCs and alcohols. However, the blowing agent compositions of Volker produce extruded foams which have poor thermal insulating properties. Specifically, none of Volker's examples show extruded foams having a coefficient of thermal conductivity higher than 0.0376 W/mK as measured by DIN 52 612.
Applicants have surprisingly discovered that by use of a blowing agent containing only 134a and alcohol, extruded foams may be produced with superior thermal insulating properties. The extruded foams made by the present invention have a coefficient of thermal conductivity lower than 0.035 W/mK and preferably lower than 0.030 W/mK as measured by DIN 52 612.